EE394V DG Fall2008 Week6 Part2

8/8/2019 EE394V DG Fall2008 Week6 Part2

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1 Alexis Kwasinski, 2008

Energy Storage

In the past 2 classes we have discussed battery technologies and how their

characteristics may or may not be suitable for microgrids.

Batteries are suitable for applications where we need an energy delivery

profile. For example, to feed a load during the night when the only source is PV

modules.

However, batteries are not suitable for applications with power delivery

profiles. For example, to assist a slow load-following fuel cell in delivering

power to a constantly and fast changing load.

For this last application, two technologies seem to be more appropriate:

Ultracapacitors (electric energy) Flywheels (mechanical energy)

Other energy storage technologies not discussed in here are superconducting

magnetic energy storage (SMES magnetic energy) and compressed air (or

some other gas - mechanical energy)


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Power vs. energy delivery profile technologies

Ragone chart:

More information and charts can be found in Holm et. al., A Comparison of

Energy Storage Technologies as Energy Buffer in Renewable Energy Sources

with respect to Power Capability.


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Power vs. energy delivery profile technologies


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Electric vs. Magnetic energy storage

Consider that we compare technologies based on energy density (J/m3)

Plot of energy density vs. length scale (distance between plates or air gap):

Hence, magnetic energy storage (e.g. SMES) is effective for large scale

systems (higher power)

[ ] [ ] [ ][ ] Energy Work F d Nm J = = = =

3 3 2[ ]

J Nm N Energy density Pa

m m m= = = =

University of Illinois at Urbana-Champaign

ECE 468 (Spring 2004)


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Ultracapacitors

Capacitors store energy in its electric field.

In ideal capacitors, the magnitude that relates the charge generating theelectric field and the voltage difference between two opposing metallic plates

with an areaA and at a distance d, is the capacitance:

In ideal capacitors:

Equivalent model of real standard capacitors:

QC

V

=

AC

d=

2 2

1w

l

ESR RR C

= +


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Ultracapacitors technology: construction Double-layer technology

Electrodes: Activated carbon (carbon cloth, carbon black, aerogel carbon,

particulate from SiC, particulate from TiC) Electrolyte: KOH, organic solutions, sulfuric acid.

Ultracapacitors

http://www.ultracapacitors.org/img2/ultraca

pacitor-image.jpg


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Ultracapacitors technology: construction

Key principle: area is increased and distance is

decreased

There are some similarities with batteries but there are

no reactions here.

Ultracapacitors

The charge of ultracapacitors, IEEE

Spectrum Nov. 2007

Traditional standard

capacitor

Double layercapacitor

(ultracapacitor)

Ultracapacitor with carbon

nano-tubes electrodes

AC

d=



Ultracapacitors technology: construction

Ultracapacitors

www.ansoft.com/firstpass/pdf/CarbonCarbon_Ultracapacitor_Equivalent_Circuit_Model.pdf



Some typical Maxwells ultracapacitor packages:

At 2.7 V, a BCAP2000 capacitor can store more than 7000 J in the volume of

a soda can.

In comparison a 1.5 mF, 500 V electrolytic capacitor can store less than 200 J

in the same volume.

Ultracapacitors




Comparison with other capacitor technologies

Ultracapacitors




Charge and discharge: With constant current, voltage approximate a linear variation due to a very

large time constant:

Temperature affects the output (discharge on a constant power load):

Ultracapacitors

www.ansoft.com/firstpass/pdf/CarbonCarbon_Ultr

acapacitor_Equivalent_Circuit_Model.pdf



Aging process: Life not limited by cycles but by aging Aging influenced by temperature and cell voltage Overtime the materials degrade, specially the electrolyte Impurities reduce a cells life.

Ultracapacitors

Linzen, et al., Analysis and Evaluation of Charge-Balancing

Circuits on Performance, Reliability, and

Lifetime of Supercapacitor Systems



Power electronic interface: It is not required but it is recommended

It has 2 purposes: Keep the output voltage constant as the capacitor discharges (a

simple boost converter can be used) Equalize cell voltages (circuit examples are shown next)

Ultracapacitors



Model (sometimes similar to batteries)

Ultracapacitors

Mierlo et al., Journal of Power Sources 128

(2004) 7689

http://www.ansoft.com/leadinginsight/pdf/High

%20Performance%20Electromechanical%20Design/Ultracapacitor%20Distributed%20Model

%20Equivalent%20Circuit%20For%20Power

%20Electronic%20Circuit%20Simulation.pdf

Ultracapacitors for Use in Power Quality and

Distributed Resource Applications, P. P. Barker



Flywheels

Energy is stored mechanically (in a rotating disc)

Flywheels Energy

Systems

MotorGenerator



http://www.vyconenergy.com

http://www.pentadyne.com

Flywheels



Flywheels

Kinetic energy:

whereIis the moment of inertia and is the angular velocity of a rotating disc.

For a cylinder the moment of inertia is

So the energy is increased if increases or ifIincreases.

I can be increased by locating as much mass on the outside of the disc as

possible.

But as the speed increases and more mass is located outside of the disc,

mechanical limitations are more important.

21

2kE I=

2 I r dm=

412

I r a =



Flywheels

Disc shape and material: the maximum energy density per mass and the

maximum tensile stress are related by:

Typically, tensile stress has 2 components: radial stress and hoop stress.

max/me K =



Since

(1)

and

(2)

and

(3)

then, from (2) and (3)

(4)

So, replacing (1) in (4) it yields

max/me K =

2

" " I r m=

2 2 21 1

2 2me r v= =

21

2kE I=

maxmax

2Kv

=

Flywheels



However, high speed is not the only mechanical constraint

If instead of holding output voltage constant, output power is held constant,then the torque needs to increase (becauseP = T) as the speed decreases.

Hence, there is also a minimum speed at which no more power can be

extracted

If

and if an useful energy (Eu) proportional to the difference between the disk

energy at its maximum and minimum allowed speed is compared with the

maximum allowed energy (Emax ) then

Flywheels

max

min

r

vV

v=

2

2

max

1u r

r

E V

E V

=

Bernard et al., Flywheel Energy

Storage Systems In Hybrid And

Distributed Electricity GenerationVr

Vr

Eu

/Em

ax



Flywheels

In order to reduce the friction (hence, losses) the disc is usually in a vacuum

chamber and uses magnetic bearings.

Motor / generators are typically permanent magnet machines. There are 2

types: axial flux and radial flux. AFPM can usually provide higher power and

are easier to cool.

Bernard et al., Flywheel Energy

Storage Systems In Hybrid And

Distributed Electricity Generation

Bernard et al., Flywheel Energy Storage Systems In Hybrid And

Distributed Electricity Generation


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Flywheels Simplified dynamic model

Typical outputs

Flywheels Energy

Systems

EE394V DG Fall2008 Week6 Part2

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